It has been known that in order to provide an effective spray apparatus for materials with a high particulate content, it is necessary to provide means for maintaining a continuous circulation of the particulate-laden liquid both during the periods when the liquid is being sprayed and during intermediate periods when the spray is interrupted. A continuous circulation of liquid serves to maintain the particulate material in suspension within the carrier liquid.
Conventional valving arrangements have proven to be unsatisfactory for fluent materials in which the particulate material is highly abrasive.